Trigger mechanism for a power tool

ABSTRACT

An electromotor-driven power tool has a trigger mechanism controlling the operation of the electromotor. The trigger mechanism comprises a trigger constructed as an one-arm lever pivotable between an inactive and an active position in which latter it activates a switch controlling the supply of electric power to the electromotor. A latch member is supported in the trigger so as to be movable with respect thereto from a latching position in which it prevents the trigger from being displaced, to an intermediate position in which it permits displacement of the trigger into the active position thereof and its immediate return into the inactive position once a pressure exerted on the trigger is relieved, and beyond the intermediate position to a locking position in which the trigger is retained in the active position until the latch member is released either by deliberate action of the operator or by an impact.

United States Patent [191 Schilling et al.

[ 1 TRIGGER MECHANISM FOR A POWER TOOL [75] Inventors: Rainer Schilling, Stuttgart;-Ladislaus Ferenczeiy, Leinfelden, both of Germany [73] Assignee: Robert Bosch G.m.b.llr, Stuttgart,

Germany 22 Filed: Dec. 12, 1973 211 Appl. No.: 422,597

[ Apr. 29, 1975 3,379,852 4/1968 Korshak 200/157 Primary Examiner-Robert K. Schaefer Assistant Examiner-William J. Smith Attorney, Agent, or Firm-'Michael S. Striker 57 ABSTRACT An electromotor-driven power tool has a trigger mechanism controlling the operation of the electromotor. The trigger mechanism comprises a trigger constructed as an one-arm lever pivotable between an inactive and an active position in which latter it activates a switch controlling the supply of electric power to the electromotor. A latch member is supported in the trigger so as to be movable with respect thereto from a latching position in which it prevents the trigger from'being displaced, to an intermediate positionin which it permits displacement of the trigger into the active position thereof and its immediate return into the inactive position once a pressure exerted on the trigger is relieved, and beyond the intermediate position to a locking position in which the trigger is retained in the active position until the latch member is released either by deliberate action of the operator or by an impact.

12 Claims, 4 Drawing Figures TRIGGER MECHANISM FOR A POWER TOOL BACKGROUND OF THE INVENTION The present invention relates to power tools in general, and to electromotor-driven power tools in particular. Even more particularly, the present invention relates to a trigger mechanism for use in an electromotordriven power tool.

There are already known various electromotordriven power tools which are provided with trigger mechanisms controlling the supply of electric current to the electromotor. In most cases, such trigger mechanisms are arranged within or adjacent the handle of the power tool so as to enable the operator of the device to control the trigger mechanism while grasping the handle of the power tool. There are also already known various trigger mechanisms which permit a selection of the mode of operation of the power tool, having two settings, in one of which the power tool is operative only as long as the operator holds the trigger device, and in the other one of which the power tool'is operative until the operator releases the trigger device.

There are also already known electromotor-driven power tools in which the danger of injuries is significantly reduced in that the trigger mechanism is so constructed as to prevent accidental putting of the power tool into operation without a willful act on the part of the user. However, experience has shown that serious injuries to the user can also occur ifa power tool having a trigger mechanism of the afore-described type is allowed to operate after the operator has ceased to act on the trigger mechanism without actually releasing the latter.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an electromotor-driven power tool having a trigger mechanism of the abovementioned type which is devoid of the disadvantages of the prior-art power tools.

More particularly, it is an object of the present invention to provide a trigger mechanism for such a power tool, whose utilization reduces the danger of injury to the operator of the power tool.

It is a further object of the invention to provide a power tool having a trigger mechanism allowing operation of the power tool in two settings, the first one of which provides for intermittent operation as long as the operator depresses the trigger mechanism, the other one being a locked setting for continuous operation until the operator releases the trigger mechanism.

It is still another object of the invention to provide a power tool having a trigger mechanism which activates the electromotor only as a result of a deliberate action on the part of the operator.

It is yet another object of the invention to provide a power tool trigger mechanism which becomes released from its locked setting in response to an impact.

It is an additional object of the present invention to provide a trigger mechanism for a power tool in which the conversion from intermittent to continuous setting is a result of deliberate action on the part of the operatOl'.

In pursuance of these objects and others which will become apparent hereinafter, one feature of the invention resides in the provision of a latch member which is supported in a trigger member so as to be movable substantially transversely to the direction of movement of the trigger member between a latching position in which it prevents the trigger member from moving, through an intermediate position in which it permits the trigger member to move into an active position so as to activate a switch controlling the supply of electric currentto the electromotor, and into a locking position in which it retains the trigger member in the active position thereof. As long as the latch member is in its latching position, accidental exertion of force on the trigger member does not result in the displacement with attendant activation of the switch. Once the latch member is deliberately moved into its intermediate position, the trigger member is permitted to move in response to a force exerted upon it by the operator so that the switch is activated and the electromotor of the power tool operates as long as the force is being exerted upon the trigger member, while the trigger member is free to return toits'original position upon discontinuance of the force, with attendant return of the latch member into its original latching position. Thus, the intermittent operation of the power tool requires deliberate displacement of the latch member from its latching position into its intermediate position and subsequent displacement of the trigger member. When it is desired to lock the trigger member in its active position, a further deliberate action on the part of the operator is required in displacing the latch member even farther with respect to the trigger member from the intermediate position into the locking position. Once the latch member is in its locking position, the pressure on the trigger member may be discontinued while the trigger member is held in its active position by the latch member.

Thus, unless the latch member is in its locking position, discontinuance of pressure on the trigger member results in deactivation of the switch and thus the electromotor of the power tool is shut off every time the power tool is laid out of hand, while any subsequent activation thereof is only possible after the latch member has been displaced into its .intermediate position. As a result of this arrangement, the danger of injury to the personnel handling the power tool during the intermittent operation of the power tool is significantly reduced, and the danger of accidental starting of the electromotor is prevented.

According to a further. feature of the invention, the latch member is supported in the trigger memberfor movement transversely of the direction of movement of the trigger member itself, and the trigger member is provided with two steppedabutments generally corresponding to the intermediate and the locking position of the latch member. Thus, during the intermittent operation of the power tool, the finger of the operator which exerts pressure on the latch member so as to displace the same from its latching into its intermediate position is prevented by the first one of the abutments from continuing its travel with respect to the trigger member beyond the position corresponding to the intermediate position of the latch member, so that accidental locking of the trigger member in its active position is prevented. When it is desired to lock the trigger member in its active position, the operator has to lift at least that finger which contacts the latch member off the trigger member so as to be able to push the latch member beyond its intermediate position toward the second abutment and thus into its locking position. Thus, the conversion from an intermittent to a continuous operation of the power tool is the result of a deliberate action of the operator. Consequently, the danger that the operator may suffer injuries as a result of the latch member being in the locking position without knowledge about this fact on part of the operator, is eliminated.

It is a further feature of the present invention that the latch member is retained in its locking position with a force which is only sufficient to retain the latch member during the normal operation of the power tool. while an impact or excessive vibrations result in return of the latch member into the intermediate position and from there into the latching position, provided no force is exerted upon the trigger member. Consequently, the supply of electric current to the electromotor is discontinued every time the power tool is subjected to external forces, such as occur when the power tool is dropped, so that injuries which may be otherwise suffered by the operator subsequently to his accidentally dropping the tool are avoided.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view ofa power tool utilizing a trigger mechanism according to the invention;

FIG. 2 is a detailed, partially sectioned view of the handle of the power tool according to FIG. 1 showing the trigger mechanism;

FIG. 3 is a detailed, partly sectioned view ofa trigger of the trigger mechanism; and

FIG. 4 is a detailed, partly sectioned view of a latch member of the trigger mechanism.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A power tool, which is illustrated in FIG. 1 as a sander but which is not limited to this particular embodiment, comprises a housing which, in a known manner,

' includes a main portion 1 accommodating an electromotor M, a head 2 connected to the main portion 1 and accommodating a transmission T for driving a tool, such as a sander plate 3, and a handle 4 which is shown in more detail in FIG. 2 and which extends from the main portion 1.

An electric switch 5 of conventional design known per se is accommodated within the handle 4 and is adapted to control the supply of electric current to the electromotor M. A trigger 6 which is constructed as an elongated one-armed lever is attached to the handle 4 so as to be pivotable in a limited range about a pin 6. A further pin 7 connects an actuating rod 7 to the trigger 6, the rod 7 being accommodated in the switch 5 and adapted to activate the same. Biasing means. such as a spring 8, is accommodated in a hole 8' provided in the trigger 6 and abuts against the switch 5 or a stationary part of the handle 4 so as to bias the trigger 6 away from the switch 5 into its normal undepressed position with attendant partial emergence of the actuating rod 7 out of the switch 5, in which position the switch 5 disconnects the electromotor M from the power source. FIG. 2 shows the trigger in its undepressed position.

The free end of the trigger 6 is provided with a guide 9 for guiding a latch member 10. In a currently pre ferred embodiment, the guide 9 comprises two lateral walls each of which is provided with a guide rail 11. The latch member 10 is provided with a pair of grooves 12 which are adapted to accommodate the guide rails 11 of the lateral walls of the guide 9. Once the guide rails 11 are inserted into the grooves 12, the latch member 10 is supported in the trigger 6 for displacement with respect thereto in a direction determined by the orientation of the grooves 12. It is to be understood that instead of being provided in the latch member 10, the grooves may also be provided in the guide 9; then, of course, the latch member 10 is formed with corresponding guide rails. Advantageously, the grooves 12, and consequently also the guide rails 11 extend substantially longitudinally of the trigger 6, so that the latch member 10 is displaceable substantially normally to the direction of movement of the trigger 6 about the pin 6'. A support plate 21 is arranged between the lateral walls of the guide 9, and the latch member 10 is provided with a bore 13 which extends parallel to the grooves 12. Biasing means, such as a spring 14 is accommodated in the bore 13 and abuts against the plate 21, biasing the latch member 10 away from the plate 21 toward a latching position of the latch member 10. The latch member 10 is further formed with a stop 15 and a recess 18 cooperating with a catch 16 which is fixed with respect to the handle 4, being attached to a stationary part thereof, such as to the switch 5. The catch 16 is preferably an angular part made of sheet material and cooperates with the latch member 10 in such a manner as to prevent inadvertent displacement of the trigger 6 when the latch member 10 is in its latching position as shown in the drawing, and thus to prevent possible injuries to the operator which could otherwise occur if the trigger 6 were permitted to move and, consequently, to activate the electromotor M and thus the sanding plate 3 every time the operator touches the trigger 6.

It may be seen in FIGS. 2 and 3 that the lateral walls of the guide 9 have a stepped configuration, being formed with a first abutment l7 and a second abutment 19, generally corresponding to an intermediate and a locking position of the latch member 10, respectively. It may be further seen in FIGS. 2 and 3 that the latch member 10 further comprises a projection 20 which extends into the recess 18. The function of these abutments and this projection will be explained hereafter.

It has been already mentioned that as long as the latch member 20 is in its latching position, the trigger 10 is prevented from being displaced in a direction resulting in actuation of the switch 5. When it is desired to start the electromotor M of the power tool, the operator has to move the latch member 10 from its latching position into its intermediate position by exerting pressure on the latch member 10 until the latter is displaced to such an extent as to register with the first abutment 17. It is evident that once the finger of the operator abuts against the first abutment 17, it is prevented from going any further. and so is the latch member 10, which is now in its intermediate position. It may be seen in the drawings that once the latch member is displaced into its intermediate position in the drawings toward the leftagainst the biasing force of the spring 14 the catch 16, instead of being opposite the stop is now opposite the recess 18, so that the trigger 6 is allowed to pivot about the pin 6' against the biasing force of the spring 8, thus forcing the actuating rod deeper into the switch 5 with attendant activation of the electromotor M. As long as the trigger 6 is in its active position. the switch is closed and the electromotor M drives the sanding plate 3, while the latch member 10 is prevented from returning into the latching position, being retained by the catch 16. However, once the pressure on the trigger 6 is relieved, the spring 8 returns the trigger into its inactive position so that the catch 16 emerges from the recess 18 and the biasing force of the spring 14 displaces the latch member 10 into its latching position. Consequently, each of the successive operations of the power tool requires separate and deliberate displacement of the latch member 10 from its latching position into its intermediate position against the biasing force of the spring 14, so that inadvertent exertion of pressure on the trigger 6 alone without first displacing the latch member 10 does not activate the electromotor M. This is particularly advantageous if the power tool is used intermittently and periods intervene during which the power tool is laid aside, in that the grasping of the handle with attendant touching of the trigger does not automatically result in activation of the electromotor M.

Whenever it is desired that the power tool be in operation for extended periods of time without the need for the operator to exert pressure on the trigger 6 for the entire duration of this extended period, the latch element 10 is pushed beyond its intermediate position into its locking position. To accomplish this, another deliberate action is required from the operator of the power tool; namely, he has to lift at least one finger from the trigger 6 so as to be able to push the latch member 10 beyond its position in which it registers with the first abutment 17 and toward the second abutment 19. In order to be able to do this, the latch member 10 has to be displaced into its intermediate position and the trigger 6 has to be displaced into its active position so that the electromotor is activated and the catch 16 extends into the recess 18. Only after these initial steps have been taken is it possible to shift the latch member 10 beyond its intermediate position into the locking position in FIG. 2 farther to the left. Once this is done, the projection 20 faces an angular end portion 16 of the catch 16 and if the pressure on the trigger is now released while the latch member is held in the locking position, the projection 20 abuts against the angular end portion 16 of the catch 16 so that the friction between the projection 20 and the angular end portion 16' of the catch 16 prevents the latch member 10 from returning into its intermediate position and from there into its latching position with attendant stoppage of the electromotor M. In a currently preferred embodiment, an additional spring 14' is attached to the plate 21 in such a manner that it abuts against the latch member 10 once the latter is displaced beyond its intermediate position, so that the biasing force the operator of the power tool has to overcome when displacing the latch member 10 from the intermediate position toward the locking position thereof increases significantly when compared with the force which he has to overcome during the displacement of the latch member 10 from the latching position into the intermediate position thereof. This is an additional-safeguard purporting to avoid the displacement of the latch member 10 into its locking position without the operator of the power tool being aware of this fact. In the currently preferred embodiment of the invention, the biasing force exerted on the latch member 10 by the two springs 14 and 14' is so selected as to be equal to or only slightly smaller than the friction force between the projection 20 of the latching member 10 and the angular end portion 16 of the catch 16. Consequently, the catch 16 safely retains the latch member 10 in its locking position under normal operating conditions until the operator decides to discontinue the operation by exerting pressure on the trigger 6 so that the projection 20 is lifted off the angular end portion 16 of the catch 16 with attendant elimination of the friction force, so that the combined action of the springs 14 and 14 shifts the latch member 10 into its intermediate position from which it is allowed to return into the latching position once the pressure on the trigger 6 is relieved. However, under unusual circumstances, such as when the power tool falls to the ground, the resulting impact, combined with inertial forces of the stationary and movable parts of the trigger mechanism, momentarily lifts the projection 20 of the angular end portion 16' of the catch 16, with attendant return of the latch member 10 into its intermediate position and from there into its latching position while the spring 8 forces the trigger 6 to return into its inactive position thus shutting off the electromotor M. In this manner, theelectromotor M driving the power tool is disactivated once the power tool gets out of the control of the operator thereof, so that the danger of damage which would otherwise exist if the power tool were operating even after it got out of control is eliminated.

While the invention has been described as incorporated in a power tool having an intermittent and a continuous setting, it could also be used in a power tool working only under intermittent conditions; in this case, the projection 20 could be omitted and the latch member would be displaceable only between a latching position and a working position corresponding to the intermediate position as described above, having still the same advantages as those previously described when discussing the latching and the intermediate positions of the latch member 10. It is also possible to dispose of the second spring 14' so that there is no stepwise increase in the force the operator has to overcome when pushing the latch member 10 beyond the intermediate position thereof, without appreciable decrease in the reliability of the device. Namely, since the operator has to lift at least one of his fingers off the trigger 6 in order to be able to push the latch member 10 beyond the intermediate position and toward the locking position thereof, this fact alone makes him aware of the situation without need to further remind him by increasing the spring force. It is also possible to provide a single spring 14 having two stiffness coefficients, the lower one being applicable during the movement of the latch member 10 from the latching into the intermediate position, and the higher one being applicable during movement beyond the intermediate position.

It is also possible to provide the angular end portion of the catch 16 with an aperture adapted to accept the projection 20. it is equally possible, if so desired, to omit the angular end portion 16' of the catch 16 entirely the catch 16 is then a strip which has been bent only onceand to provide a cutout therein which is adapted to accept a projection of the latch member 10 therein.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the types described above.

While the invention has been illustrated and described as embodied in the power tools, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is set forth in the appended claims:

1. ln a power tool having an electrically energizable drive controlled by a switch and accommodated in a housing, a combination comprising a trigger connected with the switch and mounted on the housing for displacement with respect thereto between a deenergizing position in which the switch is open and an energizing position in which the switch is closed whereby the drive is energized; biasing means urging said trigger toward said deenergizing position; a latch member mounted on said trigger to share the displacement thereof and also for movement with respect to said trigger between a first, a second, and a third position; and means on the housing operative for engaging said latch member when the same is in said first position so as to prevent displacement of said trigger toward said energizing position, for disengaging said latch member when the same is in said second position so as to permit displacement of said trigger against the force of said biasing means toward said energizing position, and for reengaging said latch member when the same is subsequently moved to said third position while said trigger is in said energizing position whereby to lock said trigger in said energizing position.

2. A combination as defined in claim 1, wherein said trigger is an elongated lever supported in the housing for pivotal movement.

3. A combination as defined in claim 2, wherein said biasing means is a spring acting on said trigger so as to pivotally displace the same toward said deenergizing position.

4. A combination as defined in claim 1, said means comprising a catch affixed to the housing and having a projecting portion; and wherein said latch member comprises a stop located opposite said projecting portion when said latch member is in said first position, and is further provided with a recess adapted to accept said projecting portion subsequently to the displacement of said latch member into said second position and of said trigger into said energizing position.

5. In a power tool having a housing including a handle and an electrically energizable drive controlled by a switch, a combination comprising a trigger movable in a first direction between a first position in which the switch is open and a second position in which the switch is closed whereby the drive is energized, said trigger being formed with a guide means comprising a first and a second abutment; biasing means urging said trigger toward said first position; a latch member supported by said guide means of said trigger for movement with respect thereto in a second direction substantially normal to said first direction between a latch position in which said latch member prevents said trigger from being displaced, a working position in which said latch member registers with said first abutment and permits displacement of said trigger into said second position in response to a pressure exerted upon said trigger, and permits the return of said trigger into said first position under the urging of said biasing means subsequently to relief of the pressure upon said trigger, and a locking position in which the latch member registers with said second abutment and subsequently to the displacement of said trigger in said second position; and means for retaining said trigger in said second position while said latch is in said locking position.

6. In a power tool having a housing including a handle and an electrically energizable drive controlled by a switch, a combination comprising a trigger in a first direction between a first position in which the switch is open and a second position in which the switch is closed whereby the drive is energized; biasing means urging said trigger toward said first position; a latch member supported by said trigger for movement with respect thereto in a second direction substantially normal to said first direction and between a latching position in which said latch member prevents said trigger from being displaced, a working position in which said latch member permits displacement of said trigger into said second position in response to a pressure exerted upon said trigger, and permits the return of said trigger into said first position under the urging of said biasing means subsequently to relief of the pressure upon said trigger; and additional biasing means urging said latch member toward said latching position thereof.

7. A combination as defined in claim 6, wherein said biasing means is a spring accommodated in said latch member and abutting against a portion of said trigger.

8. A combination as defined in claim 7, comprising an additional spring attached to said portion of said trigger and abutting against said latch member subsequently to movement thereof beyond said working position toward said locking position.

9. In a power tool having a housing including a handle and an electrically energizable drive controlled by a switch, a combination comprising a trigger formed as an elongated lever supported in the handle of the power tool for pivotal movement in a first direction between a first position in which the switch is open and a second position in which the switch is closed whereby the drive is energized; biasing means urging said trigger toward said first position; a latch member supported by said trigger for movement with respect thereto in a second direction substantially normal to said first direction and substantially coinciding with the longitudinal direction of said trigger between a latching position in which said latch member prevents said trigger from being displaced, a working position in which siad latch member permits displacement of said trigger into said second position in response to a pressure exerted upon said trigger, and permits the return of said trigger into said first position under the urging of said biasing means subsequently to relief of the pressure upon said trigger and a locking position subsequently to the displacement of said trigger into said second position; and means for retaining said trigger in said second position while said latch member is in said locking position, and comprising a catch affixed to the handle of the power tool and having a projecting portion, and a projection provided on said latch member and engaging said projecting portion when said latch member is in said locking position.

10. A combination as defined in claim 9, further comprising additional biasing means urging said latch member toward said working position and said latching position; and wherein the engagement between said projection of said latch member and said projecting portion of said catch is due to a friction force between the former and the latter, said friction force exceeding a biasing force exerted by said additional biasing means.

11. A combination as defined in claim 10, wherein said projecting portion is provided with an angular end portion, said latch member is provided with a recess adapted to accommodate said projecting portion subsequently to displacement of said latch member into said working position and of said trigger into said second position and between said working position and said locking position of said latching member; and wherein said projection extends into said recess so as to engage said angular end portion of said catch.

12. A combination as defined in claim 11, wherein said catch is an angular strip member. 

1. In a power tool having an electrically energizable drive controlled by a switch and accommodated in a housing, a combination comprising a trigger connected with the switch and mounted on the housing for displacement with respect thereto between a deenergizing position in which the switch is open and an energizing position in whicH the switch is closed whereby the drive is energized; biasing means urging said trigger toward said deenergizing position; a latch member mounted on said trigger to share the displacement thereof and also for movement with respect to said trigger between a first, a second, and a third position; and means on the housing operative for engaging said latch member when the same is in said first position so as to prevent displacement of said trigger toward said energizing position, for disengaging said latch member when the same is in said second position so as to permit displacement of said trigger against the force of said biasing means toward said energizing position, and for reengaging said latch member when the same is subsequently moved to said third position while said trigger is in said energizing position whereby to lock said trigger in said energizing position.
 2. A combination as defined in claim 1, wherein said trigger is an elongated lever supported in the housing for pivotal movement.
 3. A combination as defined in claim 2, wherein said biasing means is a spring acting on said trigger so as to pivotally displace the same toward said deenergizing position.
 4. A combination as defined in claim 1, said means comprising a catch affixed to the housing and having a projecting portion; and wherein said latch member comprises a stop located opposite said projecting portion when said latch member is in said first position, and is further provided with a recess adapted to accept said projecting portion subsequently to the displacement of said latch member into said second position and of said trigger into said energizing position.
 5. In a power tool having a housing including a handle and an electrically energizable drive controlled by a switch, a combination comprising a trigger movable in a first direction between a first position in which the switch is open and a second position in which the switch is closed whereby the drive is energized, said trigger being formed with a guide means comprising a first and a second abutment; biasing means urging said trigger toward said first position; a latch member supported by said guide means of said trigger for movement with respect thereto in a second direction substantially normal to said first direction between a latch position in which said latch member prevents said trigger from being displaced, a working position in which said latch member registers with said first abutment and permits displacement of said trigger into said second position in response to a pressure exerted upon said trigger, and permits the return of said trigger into said first position under the urging of said biasing means subsequently to relief of the pressure upon said trigger, and a locking position in which the latch member registers with said second abutment and subsequently to the displacement of said trigger in said second position; and means for retaining said trigger in said second position while said latch is in said locking position.
 6. In a power tool having a housing including a handle and an electrically energizable drive controlled by a switch, a combination comprising a trigger in a first direction between a first position in which the switch is open and a second position in which the switch is closed whereby the drive is energized; biasing means urging said trigger toward said first position; a latch member supported by said trigger for movement with respect thereto in a second direction substantially normal to said first direction and between a latching position in which said latch member prevents said trigger from being displaced, a working position in which said latch member permits displacement of said trigger into said second position in response to a pressure exerted upon said trigger, and permits the return of said trigger into said first position under the urging of said biasing means subsequently to relief of the pressure upon said trigger; and additional biasing means urging said latch member toward said latchiNg position thereof.
 7. A combination as defined in claim 6, wherein said biasing means is a spring accommodated in said latch member and abutting against a portion of said trigger.
 8. A combination as defined in claim 7, comprising an additional spring attached to said portion of said trigger and abutting against said latch member subsequently to movement thereof beyond said working position toward said locking position.
 9. In a power tool having a housing including a handle and an electrically energizable drive controlled by a switch, a combination comprising a trigger formed as an elongated lever supported in the handle of the power tool for pivotal movement in a first direction between a first position in which the switch is open and a second position in which the switch is closed whereby the drive is energized; biasing means urging said trigger toward said first position; a latch member supported by said trigger for movement with respect thereto in a second direction substantially normal to said first direction and substantially coinciding with the longitudinal direction of said trigger between a latching position in which said latch member prevents said trigger from being displaced, a working position in which siad latch member permits displacement of said trigger into said second position in response to a pressure exerted upon said trigger, and permits the return of said trigger into said first position under the urging of said biasing means subsequently to relief of the pressure upon said trigger and a locking position subsequently to the displacement of said trigger into said second position; and means for retaining said trigger in said second position while said latch member is in said locking position, and comprising a catch affixed to the handle of the power tool and having a projecting portion, and a projection provided on said latch member and engaging said projecting portion when said latch member is in said locking position.
 10. A combination as defined in claim 9, further comprising additional biasing means urging said latch member toward said working position and said latching position; and wherein the engagement between said projection of said latch member and said projecting portion of said catch is due to a friction force between the former and the latter, said friction force exceeding a biasing force exerted by said additional biasing means.
 11. A combination as defined in claim 10, wherein said projecting portion is provided with an angular end portion, said latch member is provided with a recess adapted to accommodate said projecting portion subsequently to displacement of said latch member into said working position and of said trigger into said second position and between said working position and said locking position of said latching member; and wherein said projection extends into said recess so as to engage said angular end portion of said catch.
 12. A combination as defined in claim 11, wherein said catch is an angular strip member. 